Particularly in large computer systems, there is often the need to handle a wide variety of diverse tasks in parallel. One way of meeting this requirement flexibly is to operate the computer system as a “virtual machine system”. In this case, the computer system provides a plurality of virtual computer units which together share the hardware resources of the computer system but present themselves to the user as independent computer units. In this connection, “independent” means that the individual virtual computer units can execute a wide variety of operating systems with various applications, programs or scripts, and the virtual computer units within the computer system can be addressed independently of one another. Typically, this is achieved through the use of a control device in the computer system, which control device acts as an interface between the hardware resources of the computer system and the virtual computer units. By way of example, the control device may be an operating system on which an emulator is operated which emulates the functionality of computer hardware on one or more logical interfaces. Such a logical interface then has a further operating system coupled to it which is the basis of a virtual computer unit. In this case, the operating system in the virtual computer unit and the operating system underlying the control device do not necessarily need to be of the same type. A known implementation of a computer system with one or more virtual computer units is the virtual machine system VM2000 from the company Fujitsu Siemens Computers, for example.
One important task of the control device is to distribute the hardware resources over the various virtual computer units in a computer system and, in this case, particularly the computer system's total available computation power provided by one or more processors in the computer system. Since the number of virtual computer units does not have to correlate to the number of processors in the computer system, processors are frequently assigned to the various virtual computer units only for short time slices, with this assignment changing so quickly that the user is given the impression of a continuous sequence.
To control the allocation method, the administrator of a computer system can typically provide one or more allocation parameters for the various virtual computer units, said allocation parameters being used to determine each virtual computer unit's allocated share of the total computation power by taking into account the respective computation power requirement for each virtual computer unit. By way of example, it is a known practice to control the allocation method using two parameters per virtual computer unit, a relative computation power share and an upper limit for the computation power. The computation power is allocated such that the ratio of the allocated computation power for the individual virtual computer units corresponds as far as possible to the ratio of the relevant relative computation power shares without the allocated computation power exceeding the respective prescribed upper limit. Other allocation rules based on other allocation parameters are known. Examples which may be mentioned here are allocation rules based on the allocation of priorities, or allocation rules in which the computation power used in the past is also included in the current allocation determination.
In principle, the allocated computation power for an individual virtual computer unit is a stipulation of an upper limit for the computation power which can actually be used. The computation power which is actually used is dependent on the utilization level of the applications which are being executed on the virtual computer units.
Computer service providers using a computer system of the type described frequently provide their customers with the option of being able to use one or more virtual computer units with a stipulated total computation power. The agreed total computation power must be ensured by the service provider, since it is sometimes required for the operations carried out by the customer to run smoothly. Conversely, from the point of view of the service provider it should also not be exceeded unnecessarily, since the rental is typically calculated on the basis of the total computation power.
On the basis of known allocation methods providing for the input of a maximum computation power for each virtual computer unit, each virtual computer unit rented by a customer has a maximum computation power set for it such that the total computation power rented by the customer is obtained in total. If all the virtual computer units rented by a customer are being utilized up to their maximum computation power, it is ensured that the customer also receives the computation power to which he is entitled. If one or more of the virtual computer units is not being utilized up to the maximum power, however, the customer does not receive the full computation power to which he is entitled, even though another virtual computer unit which he is operating might require more than its allocated computation power.
In the case of the known allocation methods, a system administrator would need to intervene manually at this point and match the allocated maximum computation powers for the various virtual computer units to the requirements.